This invention relates to spread spectrum communication systems and, more particularly, to a receiver for such a system which employs correlation detection means.
Spread spectrum communication systems are useful in environments where a high degree of interference may be experienced. One type of wideband or spread spectrum signal processing technique which is frequently used is the so-called pseudo-random noise or pseudo-noise (PN) system. In such a system the baseband carrier is a coded signal utilizing a long string of 1 and 0 data cells which occur in a predictable sequence but which have properties similar to random numbers. To a listener not knowing the code, the cells appear to occur in a random sequence. If the PN code has N cells (bits), the rf 3 db bandwidth of the transmitted signal is approximately N times the information bandwidth. Thus for a data channel having a bit rate of 100 KBS, use of a PN code with 256 bits produces a signal bandwidth of 25.6 MHz.
In the past it has been the practice to demodulate a PN-coded signal by multiplying the received IF component by the known PN code to obtain a cross-correlation function. When a high correlation output was obtained, indicating that the signal had been acquired, means were employed to "track" this desirable correlation state by controlling the times at which subsequent correlation operations were performed on the signal.
This system suffers from the disadvantage that it locks onto and tracks only a single signal component and if multipath components are present they are not detected. Most important, if the receiver happens to lock onto a multipath component rather than the main signal component, loss of synchronization at the receiver is highly likely to occur due to the susceptibility of the multipath to fading. This is particularly true when the transmitter and/or receiver are mobile (e.g., airborne) since the characteristics of the propagation medium are susceptible to relatively rapid change. Frequent loss of synchronization at the receiver is highly disruptive to the system and can result in loss of data unless very low throughput rates are employed.
Furthermore, when this type of receiver is used in a multiple access system where signals from several transmitting sources must be decoded simultaneously, the receiver is capable of locking onto and tracking only one signal at a time. Conferencing capability is thus not fully achievable. "Conferencing" in a communication system is the ability of a listener at a receiver to simultaneously hear the outputs generated by two or more transmitters in the network, such as occurs in a "party line" telephone hookup. Because the receiver in present spread spectrum communication systems locks onto and tracks only a single component of the received signal and excludes received components for which correlation matches occur at different times relative to the tracked signal component, conferencing is impossible unless all transmitters are synchronized to the same time base and are located exactly the same distance-from the receiver. The latter condition, of course, cannot be achieved in a practical environment, particularly one involving mobile transmitters and receivers.
An experimental system known as "RAKE" was developed in the late 1950's as an approach to resolution of the multipath problem. This system employed tapped delay lines and multiple IF correlators for performing essentially two correlations in "parallel", one for detecting "mark" bits and one for detecting "space" bits. Means were provided for integrating correlation outputs in order to realign the main and multipath signals. All of the signal processing is done using IF signals and both phase and amplitude processing was provided. The "mark" and "space" signals were transmitted orthogonally. The response of the "space" correlator to a "mark" received signal is zero, and the response of the "mark" correlator to a "space" received signal is also zero. However, besides requiring the performance of multiple parallel correlation, this system due to its use of delay lines as the basic storage element, required separate IF correlation subsystems for each tap on the delay line.
Consequently, a very large amount of complex correlation circuitry was necessary and use of the system for any kind of mobile or transportable application was impossible. In addition, the detection sensitivity of the "mark" "space" concept is approximately 4 db poorer in performance than can be achieved by coherent processing in the manner described hereinafter.